The invention relates mainly to a single piece or solid fuel element designed for use in the core of a nuclear reactor in contact with any coolant fluid such as water or a gas, to define a fast or thermal spectrum simply by adapting its dimensions.
The invention also relates to a fast spectrum boiling water nuclear reactor using fuel elements of this type. In particular, this type of reactor may be used to consume plutonium produced in existing nuclear pressurized water reactors used for the generation of electricity.
Nuclear reactors designed for the production of electrical energy commonly use square or hexagonal nuclear fuel assemblies placed side by side to form the reactor core.
In this conventional layout, each assembly comprises a bundle of fuel rods supported on a frame. Each rod is composed of nuclear fuel pellets stacked on each other inside a long tube. In xe2x80x9cUOXxe2x80x9d type assemblies, nuclear fuel pellets are formed from uranium dioxide, comprising fertile uranium 238 atoms at fissile uranium 235 atoms. In xe2x80x9cMOXxe2x80x9d type assemblies, nuclear fuel pellets are formed from a mix of uranium dioxide and plutonium dioxide. They then comprise uranium 238 atoms, plutonium 238, 239, 241 and 242 atoms and a small proportion of uranium 235.
At the moment there are limitations in nuclear fuel assemblies of this type.
These problems include their complexity and cost, handling problems resulting from their deformation under irradiation and the need to provide expansion chambers for fission gases in the top and bottom parts of rods causing an increase in the height of the core and consequently the size and cost of the reactor.
In parallel to these conventional nuclear fuel assemblies, studies and experiments have been carried out on fuel elements formed from coated fissile particles agglomerated by a carbonaceous matrix. These fuel assemblies are intended essentially for use in high temperature nuclear reactors cooled by a cooling gas such as helium.
Coated fissile particles comprise a fissile nucleus with an approximately spherical shape, coated with several successive layers comprising particularly an internal porous layer capable of containing fission gases and supporting inflation of the nucleus, and a coat of silicon carbide SiC forming a leak tight barrier for fission products. These particles are said to be of the xe2x80x9cTRISOxe2x80x9d type. Their diameter can vary from a few hundred microns to a few millimeters, depending on the manufacturing process used.
At the moment, there are two types of fuel elements in which coated particles are agglomerated in a different form by a carbonaceous matrix.
In a first type of fuel element developed in the United States of America and in France, the coated particles are agglomerated in the form of cylindrical rods that are then inserted in vertical tubular ducts provided for this purpose in graphite blocks with a hexagonal cross-section forming the core of a high temperature gas cooled reactor. The cylindrical rods are obtained by agglomerating the coated particles and a matrix based on graphite powder.
In a second type of fuel element developed in Germany, the coated particles are agglomerated in the form of balls that are compacted in bulk with graphite balls of the same dimension, to form the core of a high temperature gas cooled reactor. The balls are obtained by agglomerating coated particles and a carbonaceous matrix to form the central part of the ball, and coating this central part with a peripheral layer without any coated particles.
These assemblies are used only for high temperature gas cooled reactors with a thermal neutron spectrum. With existing technology, it is impossible to use them in water reactors.
Experimentally, it has also been proposed that bundles of rods of conventional nuclear fuel assemblies should be replaced by a set of very thin plates (1 to 3 mm thick) placed parallel to each other inside the same assembly.
In general, the use of nuclear fuel assemblies incorporating a number of thin plates parallel to each other is limited to experimental reactors.
Conventional nuclear fuel assemblies with bundles of rods, usually of the xe2x80x9cUOXxe2x80x9d or xe2x80x9cMOXxe2x80x9d type, are used in pressurized water reactors operating in thermal spectrums used at the present time for generation of electricity.
Uranium 238 atoms contained in these assemblies are fertile in the epithermal range, in other words they capture neutrons to generate plutonium atoms, for which only the 239 and 241 isotopes are fissile in the thermal domain. The result is a large production of plutonium which represents a considerable energy potential and also a large source of radiotoxicity.
The need to control the nuclear material cycles makes it necessary to use plutonium produced in existing reactors as a fuel in reactors designed specially for this purpose. Thus, nuclear reactors are now being designed in order to use this plutonium as a fuel, in other words to produce electrical energy while consuming this plutonium to reduce the volume of waste originating from existing nuclear reactors.
However, most nuclear reactors being studied for this purpose at the moment use a moderator medium, in other words a neutron decelerator, to obtain a thermal or epithermal neutron spectrum. Furthermore, for reactor control reasons and particularly in order to improve the kinetic behaviour of the core, it appears that it would be necessary to use a variable proportion of uranium 238 with the plutonium. In a reactor operating in a thermal spectrum, this creates a source of plutonium nuclei that is contrary to the purpose of the exercise.
The main purpose of the invention is a new type of fuel element adapted to industrial implementation and without the disadvantages of conventional nuclear fuel assemblies with rod bundles.
Another purpose of the invention is a new design of fuel element that can be made to produce a spectrum of fast neutrons or a spectrum of thermal or epithermal neutrons at will, simply by changing its dimensions.
Another purpose of the invention is a fuel element with an innovative design that in particular makes it possible to use the plutonium produced by existing nuclear reactors as a fuel material, using a fast spectrum which is particularly conducive for consumption of plutonium.
According to the invention, these various results are obtained by means of a single piece fuel element for a nuclear reactor formed from coated fissile particles embedded in a matrix, characterised in that it comprises several parallel plates separated by spaces and connected to each other by junction parts, the said matrix being inert to the total number of heavy nuclei (fissile and fertile) and neutral to a coolant fluid circulating in the said spaces.
It would also be possible to put the fuel assembly in a metallic duct to protect it from the coolant fluid.
Due to its monolithic or single-piece nature, a fuel element formed in this way is inherently simpler than conventional assemblies used in existing nuclear reactors.
Furthermore, fission gases released under irradiation are retained in coated particles without it being necessary to provide expansion chambers comparable to the chambers currently used with rods containing nuclear fuel pellets. The total height of the core may be significantly less than the height used in a conventional reactor.
Furthermore, for a given coolant fluid, the ratio of the thickness of the plates and the width of spaces that separate them can be changed at will to give a fast spectrum or a thermal or epithermal spectrum.
According to the preferred embodiment of the invention, the fuel element has an approximately parallelepiped external shape.
In this embodiment, the plates are approximately plane.
Furthermore, the said plates are preferably approximately vertical and a control device can be inserted in at least one of the spaces separating them, to control or shutdown the reactor.
Furthermore, the parallel plates and the junction parts are advantageously perforated at predetermined levels to form windows through which the coolant fluid can pass, which increases the exchange surface area between the fuel element and the said fluid and homogenises the coolant fluid across the entire cross-section of the reactor core.
In one advantageous embodiment of the invention, the horizontal cross-section of the space in which a control device can be placed is in the shape of a cross and divides the fuel element into four sub-assemblies each comprising several plates connected to each other by some of the junction parts, other junction parts connecting the sub-assemblies together, at the periphery of the fuel element. A second control device independent of the first and also in the shape of a cross, may be placed between four adjacent fuel elements.
In another advantageous embodiment of the invention, a first space in which a first plate-shaped control device can be inserted is formed in a central part of the fuel element. A second series of spaces opening up into the periphery of the said element is then formed on at least one side of the central part, and a second rake-shaped control device can be placed in these spaces.
In general, the thickness of the plates and the width of the spaces are preferably uniform.
In the case described in which the coolant fluid is water, the ratio of the thickness of the plates and the width of the spaces is equal to 1, which will define a fast spectrum.
Preferably, coated particles comprise nuclei of fissile bodies chosen in the group including uranium and plutonium. A mix of particles based on plutonium and uranium respectively, and a choice of dimensions to enable operation in fast spectrum, are used to consume plutonium while assuring satisfactory control of the reactor.
The inert matrix is made from a material offering a low effective absorption cross-section with regard to neutrons and a high thermal conductivity.
Another purpose of the invention is a boiling water nuclear reactor comprising a core containing several fuel elements made as defined above, the ratio between the thickness of the plates and the width of spaces being a compromise between the constraints related to moderation of neutrons and extraction of thermal power. This ratio should be equal to approximately 1 to obtain a fast spectrum.